1. Field of the Invention
An apparatus includes a laser offset in wavelength from a specific wavelength and a heater that maintains the laser wavelength in a required range over a wide temperature variation.
2. Discussion of the Related Art
Wavelength Division Multiplexing (WDM) is a technology that funnels wavelengths from different sources at different bit rates and different protocols (such as Fiber Channel, Ethernet and Asynchronous Transfer Mode (ATM)) onto an optical fiber.
Each data channel, or signal, is carried on its own private and secure color of light, or wavelength. A wavelength is usually expressed in nanometers. Using WDM technology, from two to more than 80 separate wavelengths of data can be multiplexed into a light stream transmitted on one optical fiber. However, providing a device that multiplexes the multiple separate wavelengths while achieving a resultant wavelength stability throughout a wide temperature range is difficult because each wavelength of the Coarse Wave Division Multiplexing (CWDM) lasers drifts when an ambient temperature deviates from a prescribed temperature (around 25° C.) and the quality of the signal deteriorates below a required threshold. Thus, the CWDM link does not function properly at outside temperatures far from the prescribed temperatures (below −5° C. or above +55° C.). An overview of CWDM lasers and fiber optic video transmission can be found in (1) CWDM and OEO Transport Architectures, Conference Publication “Future Challenges and Opportunities for DWDM and CWDM in the Photonic Networks,” IEE Midlands Communications Group, Jun. 11, 2004, and (2) Fiber Optic Video Transmission: the complete guide, by D. Goff, Focal Press, 2003. Both of the above noted references are filed together with this application and the entire contents of the references are incorporated herein by reference.
On the receiving side, each channel is then demultiplexed back to the source wavelength. This is the same for all WDM systems, whether they are based on CWDM or Dense Wavelength Division Multiplexing (DWDM) technology.
The differences between CWDM and DWDM systems can be explained by outlining the major components of all WDM systems. These are:                An optical laser (transmitter).        An optical detector (receiver).        Optical filters for multiplexing (add) and demultiplexing (drop).        Optical amplifiers for distance extension.        
Typically, the optical laser is used for transmitting a signal and the corresponding detector is used to receive the signal on the same wavelength that was transmitted by the optical laser. In this situation, the wavelength of the laser matches the accepted wavelength range of the receiving system, which consists of optical bandpass filters and broadband detectors. The actual amount of information that is transmitted on a single wavelength is determined by the bit rate of the laser, or the bandwidth of an analog transmission.
The CWDM laser has a specified working ambient temperature (usually 25° C.) for which the laser produces the wavelength (1550 nm for example) with a certain tolerance (3 nm for example), as shown in FIG. 1(a). However, as the temperature of the ambient in which the laser operates varies from low temperatures to high temperatures (for example in the range of −30° C. to +85° C.), the CWDM laser wavelength changes (to a range of 1540.4 nm to 1560.2 nm for the above noted temperatures and a laser centered on 1550.0 nm) as shown in FIG. 1(b).
As shown in FIG. 1(f), an exemplary CWDM Optical Passive component can extract a signal that corresponds to a wavelength of 1551.0 nm if the wavelength is maintained in the range of 1544.5 nm to 1557.5 nm. That is, the CWDM component has a tolerance of +/−6.5 nm with 1 nm offset passband. Thus, if the signal produced by the CWDM laser has a wavelength that is outside the above noted range because of the change in the ambient temperature, the CWDM optical component would not be able to extract the signal, and the use of the CWDM component is drastically limited. The DWDM components tend to behave similar to the CWDM components, but require much tighter temperature control due to more closely spaced wavelengths.
DWDM transceivers also tend to increase the associated operational expenses by consuming more power and dissipating more heat than the CWDM transceivers. This increased heat dissipation causes significant operational problems for optical networks, as discussed above. In essence, a conventional CATV system that uses CWDM components would not be able to transmit the signal to the receiver, which is usually at the headend, when the ambient temperature is outside a given range, typically −5° C. to 55° C. As is known, various parts of U.S. experience temperatures outside that range. Thus, a need exists to provide CATV signal in those areas affected by extreme temperatures.
Passive optical components of a CATV system are optical couplers, optical multiplexers/demultiplexers and Optical Add Drop Multiplexers (OADM). These devices are used to manipulate wavelengths. To transmit data, optical passives take in various optical input source wavelengths and select specific wavelengths that are added to the WDM network. Optical passives are responsible for manipulating wavelengths in a fiber optic system. These functions are not limited to adding, dropping and combining signals.
The cost of a DWDM optical passive is approximately two to three times the cost of a wider-band CWDM optical passive because a DWDM environment has a significantly smaller space between wavelengths (100-GHz typical spacing) than that used in CWDM systems (approximately 2,500 GHz). This demands tight component temperature control, resulting in added cost.
On CWDM-based systems, the wavelength separation between each color of light on the fiber is significantly farther apart, or wider (by a factor of 20) than on DWDM systems. DWDM systems multiplex a large number of individual wavelengths into one fiber by using less space between each wavelength. Metropolitan DWDM systems readily support 32 and 64 wavelengths on a single fiber, and long-haul DWDM systems are typically higher densities. The standard frequency grid for DWDM and wavelength grid for CWDM systems are defined by the International Telecommunications Union standards G.694.1 and G.694.2, respectively.
CWDM is well-suited for applications that have lower data-capacity requirements and for fiber spans that are 50 km or less. This is the typical requirement for metropolitan-to-enterprise-edge applications, where the fiber distances tend to be shorter (less than 50 km). As a result, lower-cost CWDM systems provide more economic benefits while providing the same security, reliability and quality as a DWDM system.
However, conventional CWDMs are subject to intrinsic wavelength drift when operated in temperature ranges lower than −5° C. and higher than +55° C. as discussed above. More specifically, conventional CWDMs exhibit a wide wavelength tolerance as shown in FIG. 1(b) for a typical operating temperature range of the lasers, and the wide wavelength tolerance exceeds the wavelength tolerance required by the CWDM components shown in FIG. 1(f) for example for properly functioning. Therefore, applications that use CWDMs and are exposed to harsh ambient temperatures tend to malfunction, i.e., lack quality or completely cease to function because the mismatch between the wavelength tolerance of the laser and of the passive components.